1) Field of the Invention
This invention relates generally to mooring and fluid transfer systems and in particular to Floating Production Storage and Offloading vessels (FPSO's) including those for LNG liquefaction production and storage. More particularly, this invention relates to tandem offloading of a permanently moored LNG liquefaction and storage vessel to a shuttle or LNG carrier vessel. The term “tandem offloading” describes an arrangement where the shuttle vessel is behind and generally inline with the FPSO, as opposed to “side-by-side offloading” where the LNG carrier is moored along side the FPSO in a parallel position.
2) Description of the Prior Art
Periodically LNG carrier vessels arrive at the location of an LNG/FPSO to load liquefied gas for transport to distant ports. The term LNG is an acronym for Liquified Natural Gas. Highly reliable and safe temporary mooring equipment is required to mechanically connect the LNG carrier (LNGC) to the stem of the LNG/FPSO in offshore sea conditions while LNG transfer occurs between the two vessels.
FIGS. 1 and 2 illustrate a prior art LNG transfer system, such as the FMC Technologies BTT system, with piping and flexible joint swivels connecting the FPSO vessel 1 to LNG carrier vessel 2. Hawser 8 endures the mooring force to hold vessel 2 to the stern of FPSO vessel 1. Disadvantages of the hawser mooring system include the lack of restraint to prevent vessel 2 from surging forward and colliding with FPSO vessel 1. In addition, hawser 8 allows a wide range of lateral motion of vessel 2, as indicated by motion arrows L. Piping pantograph 5 is flexible and allows limited horizontal motion of LNG manifold connector 7, such as within a circle of 12 meters radius. As vessel 2 sways laterally, crane boom 4 mounted on pedestal 34, must rotate automatically to follow the wide excursions of LNGC 2 bow B while connected to manifold 7 on LNGC 2.
FIGS. 1 and 2 illustrate that because of the wide lateral movement of the LNGC 2 with respect to the end of the FPSO 1, a crane pedestal 34 with a rotatable boom 4 is required, because the pantograph 5 with a manifold connector 6 is capable of only a limited lateral movement L. It would be desirable to eliminate the crane pedestal 34 and rotatable boom 4 in favor of a fixed structure where a mooring system ensures that only limited lateral movement of the LNGC 2 with respect to FPSO 1 is possible under designed environmental forces on the vessel.
3) Identification of Objects of the Invention
A primary object of the invention is to provide an improved yoke and linkage design so that side-to-side relative motion (i.e., sway motion) between an LNG/FPSO and an LNG/shuttle tanker is greatly reduced from that of other yoke connecting arrangements. Reduction of side-to-side sway motions is highly beneficial to the LNG transfer system connected between the two vessels. The LNG transfer system will have higher reliability, greater safety, and lower cost as a result of reduced relative vessel motions.
Another object of the invention is to provide an improved disconnectable mooring device to connect an LNG/shuttle tanker or carrier to the LNG storage vessel that is intended for frequent connection and disconnection of the LNG carrier vessel in an offshore environment of at least Hs 2 meters wave height that causes relative motion between the two vessels.
Another object of the invention is to provide a disconnectable mechanical connection linkage that reduces the relative motions in the transverse direction to the FPSO vessel's longitudinal axis while not becoming too stiff and causing high forces in the fore-and-aft directions.
Another object of the invention is to provide a disconnectable mechanical connection linkage that has at least half as much resistance to lateral force (force stiffness) at the yoke tip connector as it has in the fore-and-aft vessel direction. Preferably, the linkage will be designed and arranged for a lateral resistance to force equal to or greater than the resistance in the fore-and-aft direction.
Another object of the invention is to provide a disconnectable mechanical connection linkage that effectively decouples the force stiffness in the lateral direction from the force stiffness in the fore-and-aft vessel direction.
Another object of the invention is to provide a disconnectable mechanical connection linkage whereby the force resistance in the carrier vessel's fore-and-aft direction is not greatly increased when the yoke tip and carrier vessel's bow connector has been displaced to an extreme position to one side. This action reduces the maximum linkage forces that occur at the extreme lateral displacements.
Another object of the invention is to provide an alternative disconnectable mechanical connection linkage whereby the fore-and-aft force stiffness is greater when the yoke is displaced sternward than it is when the yoke is displaced forward of its neutral position.
Another object of the invention is to provide an LNG transfer system to work in conjunction with conventional crane and boom fluid transfer arrangements with disconnectable mechanical connection linkages that, as a result of the reduced lateral relative motions of the LNGC, does not require rotation of the LNG transfer system boom about a vertical axis to follow the lateral motions of the LNGC vessel while the piping pantograph is connected to the LNGC.
Another object of the invention is to provide an LNG transfer system wherein a crane pedestal is located at a point outboard of the yoke links to achieve a minimum boom length for a given separation distance between the connected vessels.
Another object of the invention is to provide an alternative arrangement where a crane and boom assembly is eliminated in favor of a fixed cantilevered frame at the end of the FPSO with a pantograph coupling at the end of the frame.